A heat developable light-sensitive material (or a photothermographic material) has already been proposed, and is described in U.S. Pat. Nos. 3,152,904, 3,457,075, and B. Shely "Thermally Processed Silver Systems" (Imaging Processes and Materials, Neblette eighth edition, edited by Sturge, V. Walworth and A. Shepp, page 2, 1996).
The heat developable light-sensitive material generally has a light-sensitive layer, which contains a catalytically active amount of a photo catalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., organic silver salt) and a color toning agent dispersed in a binder matrix. The color toning agent has a function of controlling color tone of silver. A heat development image forming process comprises steps of imagewise exposing to light the heat developable light-sensitive material, and then heating the light-sensitive material at an elevated temperature (not lower than 80.degree. C.) to cause an oxidation-reduction reaction between the silver halide or the reducible silver salt (which functions as an oxidizing agent) and the reducing agent. Thus a black silver image is formed. The oxidation-reduction reaction is accelerated by a catalytic function of a silver halide latent image formed at the exposing step. Accordingly, the black silver image is formed within the exposed area.
The heat development does not require processing solutions of a wet development. The heat development can easily and rapidly be conducted, compared with the wet development. However, the wet development is still a major photographic technology, while the heat development is minor. The heat development has unsolved problems, while the wet development does not have the problems.
A photographic material usually contains a dye, such as a filter dye, an antihalation dye or an antiirradiation dye. The dye functions at the exposing step. If the dye remains in the photographic material after image formation, a formed image would be colored with the dye. Therefore, the dye should be removed from a photographic material at a developing step. At the wet development, the dye can easily be removed from a photographic material by using processing solutions. On the other hand, it is very difficult (substantially impossible) to remove the dye from a photographic material at the heat development.
A simple, easy and rapid development has been desired in the field of recent photography, especially in the field of recent clinical or printing photography. The improvement of the conventional wet development, however, has nearly reached its limits. Therefore, much attention has been paid again to a heat development image forming process in the field of clinical or printing photography.
Since it is very difficult to remove a dye at the heat development, it has been proposed to decolor the dye at the heat development. For example, U.S. Pat. No. 5,135,842 discloses a method of decoloring a polymethine dye of a specific structure by heating a photographic material. U.S. Pat. Nos. 5,314,795, 5,324,627 and 5,384,237 disclose a method of decoloring a polymethine dye by heating a photographic material in the presence of a carbanion forming agent (nucleophilic agent).